The size of seeds and organs is an agronomically and ecologically important trait that is under genetic control (Alonso-Blanco, C. PNAS USA 96, 4710-7 (1999); Song, X. J. Nat Genet 39, 623-30 (2007); Weiss, J. Int J Dev Biol 49, 513-25 (2005); Dinneny, J. R. Development 131, 1101-10 (2004); Disch, S. Curr Biol 16, 272-9 (2006); Science 289, 85-8 (2000); Horiguchi, G. Plant J 43, 68-78 (2005); Hu, Y Plant J 47, 1-9 (2006); Hu, Y. Plant Cell 15, 1951-61 (2003); Krizek, B. A. Dev Genet 25, 224-36 (1999); Mizukami, Y. PNAS USA 97, 942-7 (2000); Nath, U. Science 299, 1404-7 (2003); Ohno, C. K. Development 131, 1111-22 (2004); Szecsi, J. Embo J 25, 3912-20 (2006); White, D. W. PNAS USA 103, 13238-43 (2006); Horvath, B. M. Embo J 25, 4909-20 (2006); Garcia, D. Plant Cell 17, 52-60 (2005). The final size of seeds and organs is constant within a given species, whereas interspecies seed and organ size variation is remarkably large, suggesting that plants have regulatory mechanisms that control seed and organ growth in a coordinated and timely manner. Despite the importance of seed and organ size, however, little is known about the molecular and genetic mechanisms that control final organ and seed size in plants.
The genetic regulation of seed size has been investigated in plants, including in tomato, soybean, maize, and rice, using quantitative trait locus (QTL) mapping. To date, in the published literature, two genes (Song, X. J. Nat Genet 39, 623-30 (2007); Fan, C. Theor. Appl. Genet. 112, 1164-1171 (2006)), underlying two major QTLs for rice grain size, have been identified, although the molecular mechanisms of these genes remain to be elucidated. In Arabidopsis, eleven loci affecting seed weight and/or length in crosses between the accessions Ler and Cvi, have been mapped {Alonso-Blanco, 1999 supra}, but the corresponding genes have not been identified. Recent studies have revealed that AP2 and ARF2 are involved in control of seed size. Unfortunately, however, ap2 and arf2 mutants have lower fertility than wild type (Schruff, M. C. Development 137, 251-261 (2006); Ohto, M. A. PNAS USA 102, 3123-3128 (2005); Jofuku, K. D. PNAS USA 102, 3117-3122 (2005)). In addition, studies using mutant plants have identified several positive and negative regulators that influence organ size by acting on cell proliferation or expansion {Krizek, B. A. Dev Genet 25, 224-36 (1999); Mizukami, Y. Proc Natl Acad Sci USA 97, 942-7 (2000); Nath, U. Science 299, 1404-7 (2003); Ohno, C. K. Development 131, 1111-22 (2004); Szecsi, J. Embo J 25, 3912-20 (2006); White, D. W. PNAS USA 103, 13238-43 (2006); Horvath, B. M. Embo J 25, 4909-20 (2006); Garcia, D. Plant Cell 17, 52-60 (2005). Horiguchi, G. Plant J 43, 68-78 (2005); Hu, Y Plant J 47, 1-9 (2006) Dinneny, J. R. Development 131, 1101-10 (2004)).
Several factors involved in ubiquitin-related activities have been known to influence seed size. A growth-restricting factor, DA1, is a ubiquitin receptor and contains two ubiquitin interaction motifs (UIMs) that bind ubiquitin in vitro, and da1-1 mutant forms large seeds by influencing the maternal integuments of ovules (Li et al., 2008). Mutations in an enhancer of da1-1 (EOD1), which encodes the E3 ubiquitin ligase BIG BROTHER (BB) (Disch et al., 2006; Li et al., 2008), synergistically enhance the seed size phenotype of da1-1, indicating that DA1 acts synergistically with EOD1/BB to control seed size. In rice, a quantitative trait locus (QTL) for GRAIN WIDTH AND WEIGHT2 (GW2), encoding an E3 ubiquitin ligase, controls grain size by restricting cell division (Song et al., 2007). A GW2 homologue in wheat has been identified (Ta-GW2; Bednarek et al 2012). An unknown protein encoded by rice qSW5/GW5 is required to limit grain size in rice (Shomura et al., 2008; Weng et al., 2008). GW5 physically interacts with polyubiquitin in a yeast two-hybrid assay, suggesting that GW5 may be involved in the ubiquitin-proteasome pathway (Weng et al., 2008). However, it is not clear whether these two factors act in maternal and/or zygotic tissues in rice.
Identification of further factors that control the final size of both seeds and organs will not only advance understanding of the mechanisms of size control in plants, but may also have substantial practical applications for example in improving crop yield and plant biomass for generating biofuel.